In a very similar way to how Muhammad made everyone in history sound like him, and say the same things as him in the Quran, a survey of the New Testament reveals that the author of the Gospel of John put his own language and theology into the mouth of Jesus.
When reading the Synoptic Gospels (Matthew, Mark, Luke), Jesus comes across as a rather straightforward teacher, a herald of a "Kingdom of Heaven", and one who taught using many parables and pithy sayings.
However, the Jesus presented in the Gospel of John is an entirely different figure. John's Jesus has a theme of wanting to confuse his audience in ways and events never seen in the Synoptics (John 2:18-20, 3:1-10, 4:7-15, 4:31-34, 6:41-60, 7:33-36, 8:51-53). And, he uses language that is never seen in the Synoptics, but is seen heavily in the writings of "John", specifically, First John. Additionally, his theology echoes John's theology.
What follows are examples of "John" putting his own theology and language into the mouth of Jesus:
When one reads First John, the reason for the stark contrast between the Synoptic Gospels and the Gospel of John becomes obvious. Whoever "John" was, he felt content to use the historical figure of Jesus as a mouthpiece for his own theology/philosophy, and created a narrative in which Jesus is represented as essentially teaching what John wanted taught, saying many things totally unknown to any other source, and using language used extensively in John's own writings.
Many Christians (especially Christian scholars) recognize what John did, and are quick to supply a "genre" defense. That is, they contend, it was normal in Jesus's day to reframe a popular figure as teaching one's own doctrines, or to otherwise repurpose a popular figure to serve one's own agenda.
However, this defense runs into various problems. Firstly, I dispute that it was ever seen as "okay" to do this. I do not believe that if I had gone back to the first century, and written a gospel in which Jesus is teaching Buddhism, and speaking like Buddha, that no one would have strongly objected. I also dispute that if I wrote an admitted forgery putting words into the mouth of Moses, that any Jews present would have shrugged, acknowledging that I am writing in what can be called a "forgery genre". The reason so much pseudepigrapha (e.g. Ecclesiastes) was accepted in ancient times was because people were genuinely duped into believing that the authors wrote those works, even if the actual author soothed his conscience with some form of a "genre" defense.
Secondly, such a "genre", if it were ever accepted in any society, would be detrimental to it. It muddles and obfuscates historical figures and events, making it a poisonous genre.
Thirdly, it is very clear that early Christians treated the Gospel of John as representing the literal words of Jesus. So, if the author had intended to teach as "Jesus" to his original audience with their full knowledge of what he was doing, that was quickly lost, and immediately the position that won out was a strictly literal understanding (i.e., that this Gospel represented actual words that Jesus himself spoke). As a digression, the same is true for "genre" defenses of the book of Genesis. I dispute the idea that practically anyone living in Jesus's time would have given a modern "genre" view of the book of Genesis, regardless of its author(s)' intention. They (including the man Jesus) would have believed it as the strictly literal history of the world and their people, and would be highly offended by modern apologists' assertions that the whole story is fake, and in the "genre" of ancient Near-Eastern myth (which it undoubtedly is).
Finally, it can be stated on the principles of honesty that anyone who at any time put their own words into the mouth of a historical figure who never actually said them is a liar. If another time permitted or tolerated lying, that has nothing to do with whether I or anyone else today should tolerate it.
Unfortunately, the forgery that is the Gospel of John bears considerable weight in modern Christianity, despite it being the work of a person who was essentially an esoteric Jewish philosopher, attempting to bring his own strange theology into the Jesus movement.
Using someone else, especially a famous religious figure, as a puppet for one's own theology is dishonest. However, that is unfortunately what the author of 1-3 John did with Jesus of Nazareth.